If you've ever played StarCraft, you'll immediately recognize the living city in Paolo Bacigalupi's "Pocketful of Dharma" for what it is: a monstrous organic hive from which a new race of insectoid humans will soon be spawned. That was the first mental image that popped into my head, at least. In one of Bacigalupi's many disturbing future-scapes, a wasted, trash-ridden Chengdu is being consumed by a new city, one that "drew nutrients and minerals from the soil and sun, and the water of the rancid Bing Jiang; sucking at pollutants as willingly as it ate the sunlight which filtered through twining sooty mist".

Okay, maybe I came on a little strong with the Zerg-hive analogy. Why not read it and decide for yourself what the organic cities of the future might look like? (It's like ten pages, and totally rad, I promise.) For now, let's move on to the Big Science Question: could that work? Could we actually engineer living tissue to grow into something as large and complex as a city? And if living cities are possible, will they save us from the looming environmental apocalypse?

Growing Big

Firstly, there is a practical challenge to bigness. Generally speaking, as organisms get larger, their surface area : volume ratio decreases. They get fatter. As volume increases relative to surface area, it gets harder for your body to transport all the stuff your cells need- oxygen, nutrients, carbon- in a timely manner. Consequently, larger organisms tend to grow more slowly and expend less energy. For most life on Earth, the [larger = fatter = slower] identity imposes a fundamental constraint on size. This is also why we don't find ourselves living amidst hordes spherical, elephant-sized mice (a situation I think we can all agree would be simply intolerable).

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However, one group of organisms has found an evolutionary niche in getting big but staying thin: fungi. Fungi do this by producing mycelia: thread-like appendages, often a single cell thick, that can spread out over an enormous surface area. In nature, mycelia-producing fungi often associate with plants, reaching into tiny spaces that roots can't access, sucking out nutrients and trading them for sugar. Because of their extraordinarily high surface area: volume ratio, mycelia can essentially keep growing forever. Small wonder the largest organism in the world is a 2400 acre mycelial fungusthat supports entire forests in eastern Oregon.

If we can agree that using a mycelia-producing fungus as our prototype would allow us to overcome the physiological challenge of bigness, the next question is one of aesthetic.

Will our future cities be architectural masterpieces, the ultimate tribute to our creative ingenuity? Or will they be more reminiscent of our childhood drip castles, a sort of shapeless monstrosity that will be cast into the history books as an avant-garde reflection on the chaotic nature of our existence?

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Getting living tissue to grow in a complex, man-made design is not a trivial problem. Most organisms look the way they do because billions of years of evolution (or six thousand, depending on your math) made it so. Now, if we were dealing with an animal or even a plant, something with a relatively fixed body layout, we'd have to figure some way of hard-wiring the design into the organism's DNA. This would not only require enormous advancements in bioengineering, it would significantly limit our creative freedom. Once the genetic blueprint for a city was written, it would be a sorry grad student's PhD project to make any changes without screwing the whole thing up.

Once again, the solution may lie in Kingdom Fungi. Fungi do not always grow according to a genetically-ordained plan. Rather, they can alter their grow patterns based on environmental cues, such as moisture, temperature and nutrients. Even better, some fungi produce biofilms- colonies that grow along a surface, gluing themselves together by secreting a matrix of extracellular polysaccharides (sugary goop). Once formed, biofilms can be extremely resilient to physical and chemical damage.

Could the biofilm concept be applied to living city design? It would certainly simplify the engineering. Rather than attempting to rewire a huge mess of genes that influence growth and body form, we could focus on optimizing the material properties of our city-tissue. And on ensuring it loves to grow on whatever surface (Styrofoam coffee cups??) we choose as our skeleton.

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Environmental benefits

Perhaps I've convinced you that creating a living city is not outside the realm of possibility. What of the environmental benefits? Now I'm really going to go off the speculative deep end. What if city-tissue could be engineered to regulate its temperature, keeping buildings cool in the summer and warm in the winter? What if our city sucked CO2 out of the atmosphere to feed itself? What if the city's tissue itself was edible, engineered to contain all essential vitamins and amino acids? (A nifty way to ensure urban food security, perhaps, but we'd have to enforce some pretty strict regulations dictating where you can and cannot graze the side of a building). What if our city could also purify dirty water? Eat our waste? Produce anti-bacterial compounds that reduce the spread of infectious disease?

My conclusions? While living cities are still very much in the realm of speculation, the potential benefits are compelling enough to merit some government funding. And hey, vertical farmsand stem cell burgers are both promising starts.

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You can get "Pocketful of Dharma" and other awesome stories by Paolo Bacigalupi here.